A planetary gear assembly comprises planetary gear trains, each of which includes a sun gear, a pinion, a carrier, and a ring gear. In addition to these rotational components, the assembly further includes clutches and brakes to control the rotations of the components in predetermined manners. Each clutch engages a specific rotational component with another component, and each brake holds a specific rotational component stationary with respect to the housing of the transmission (transmission casing). In this planetary gear assembly, when a clutch or a brake is being brought into engagement, frictional heat is generated in the friction discs that constitute the respective engaging elements. For this reason, lubrication oil must be supplied not only to the gears but also to the clutches and brakes.
For the lubrication of components of a transmission such as gears, it is commonly practiced that an oil supply passage is formed in the input shaft of the transmission while the openings or outlets are drilled radially in communication with the supply passage to lead the oil for lubrication. However, since it is typical that the input shaft is mounted or covered with cylindrical members, such as sleeves, these cylindrical members must also have oiling apertures to deliver the oil further outward. Therefore, additional oiling apertures are provided in the cylindrical members, in alignment with the outlets of the oil supply passage. In this case, generally, sumps (concave grooves) are provided at the inlets of the oiling apertures (inside the cylindrical members) so that a small amount of shift of the cylindrical members with respect to the input shaft will not affect the flow of the lubrication oil into the oiling apertures.
However, in the above method, if the cylindrical members, e.g., sleeves, are designed in thin cylinders and are provided with such oil sumps and oiling apertures, the cylindrical members may be subjected to problems of stress concentration around the oiling apertures and of deterioration of overall strength because of the reduced thickness caused by the formation of the sumps. On the other hand, if these cylindrical members are designed relatively thick to prevent such problems, then the weight and size of the planetary gear assembly may increase by a certain amount, making the miniaturization of the assembly difficult, which is an alternative problem.